1. Field of the Invention
The present invention relates to an inverter circuit for controlling illumination of a discharge lamp.
2. Description of the Related Art
FIG. 1 designates a conventional inverter circuit of a discharge lamp. When the AC power is ON, transistors Q1 and Q2 of this inverter circuit are alternately turned ON and OFF in response to the output current from secondary windings N21 and N22 of a drive transformer DT, where operative timing of these transistors Q1 and Q2 is controlled by a time constant circuit which is composed of transistors Q3 and Q4 respectively being connected to the secondary windings N21 and N22, resistor R4, capacitor C5, resistor R6, and capacitor C6 respectively connected to bases of these transistors Q3 and Q4.
In response to the ON/OFF operations of the transistors Q1 and Q2, a capacitor C and a choke coil CH provided between filament electrodes Fl and F2 of a discharge lamp L respectively resonate themselves to cause the filament electrodes Fl and F2 to be pre-heated so that a high voltage can be generated at both terminals of the capacitor C. In consequence, the discharge lamp L lights up in a very short period of time after the power is ON.
Generally, in order to stably light up a discharge lamp, it is essential that the negative characteristic of the discharge lamp and the negative characteristic of the inverter circuit can cross each other at a sufficient angle.
Nevertheless, as shown in FIG. 2A, if it is so arranged that a constant-current characteristic be added to the negative characteristic INC of the inverter circuit, preheating secondary voltage tends to rise too high in the inverter circuit of the type mentioned above. In an extreme case, due to increased resonant current flowing through the inverter circuit, the inverter circuit itself may be destroyed.
On the other hand, if it is so arranged that the preheating secondary voltage be set to an optimal level, as shown in FIG. 2B, the negative characteristic INC of the inverter circuit deviates from the constant-current characteristic, and as a result, the negative characteristic INC cannot stably cross the negative characteristic LC of the discharge lamp. This in turn causes the discharge lamp to either flicker or turn OFF itself.